This invention relates to the production of thin steel strip in a strip caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which molten metal flows through a metal delivery nozzle located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668; 5,277,243; and 5,934,359.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures of the order of 1400° C. and if exposed to air, the strip suffers very rapid scaling due to oxidation of the strip at such high temperatures.
It has therefore been proposed to shroud the newly cast strip within an enclosure containing a non-oxidizing atmosphere until its temperature has been reduced significantly, typically to a temperature of the order of 1200° C. or less so as to reduce scaling. One such proposal is described in U.S. Pat. No. 5,762,126 according to which the cast strip is passed through a sealed enclosure from which oxygen levels are reduced by initial oxidizing of the strip passing through the enclosure. Thereafter the oxygen content in the sealed enclosure is maintained at less than the surrounding atmosphere by continuing oxidizing of the strip passing through the enclosure and controlling the thickness of the scale on the strip emerging from the enclosure. The emerging strip may be reduced in thickness in an in-line rolling mill and then generally subjected to forced cooling, for example by water sprays, and the cooled strip is then coiled in a conventional coiler.
As more fully described in U.S. patent application Ser. No. 09/967163 and International Application PCT/AU01/01215, steel strip can be produced from molten steel of a given composition with any of a wide range of microstructures, and in turn a wide range of yield strengths, by continuously casting the strip and thereafter selectively cooling the strip to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C. It is understood that the transformation range is within the range between 850° C. and 400° C. and not that entire temperature range. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
Specifically, from work carried out on low carbon steel, including low carbon steel that has been silicon/manganese killed or aluminum killed, it has been determined that selecting cooling rates in the range of 0.010° C. /sec to greater than 100° C. /sec, to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C., can produce steel strip that has yield strengths that range from 200 MPa to 700 MPa or greater. By selection of an appropriate cooling rate it is possible to produce a microstructure which governs the yield strength selected from a group that includes microstructures that are (1) predominantly polygonal ferrite; (2) a mixture of polygonal ferrite and low temperature transformation products and (3) predominantly low temperature transformation products. The term “low temperature transformation products” includes Widmanstatten ferrite, acicular ferrite, bainite and martensite.
This development enables production of thin steel strip from molten steel of a given chemistry to meet differing customer-specified yield strength requirements by varying the conditions under which the as-cast strip is cooled through the austenite to ferrite transformation range.
As described in U.S. application Ser. No. 60/236390, it is also possible to change other process parameters in the strip casting process to produce strip meeting varying customer requirements from a given strip casting line.
By the present invention, the thickness of the as-cast strip is controlled by changing the depth of the casting pool. This enables the casting rolls to be operated at a generally constant heat flux, which permits maximum throughput without generating excessive temperatures at the casting surfaces while varying the strip thickness. Accordingly, a single-roll profile may be used for casting rolls with a substantially constant throughput to produce a broad range of different cast strip thicknesses. Also, with the present invention, a constant as-cast microstructure can be maintained in the cast strip, which can consistently and predictably be modified and controlled by the subsequent cooling regime to produce strip having customer-specified properties. Further, increased flexibility in varying the thickness of the as-cast strip is provided that enables the subsequent reduction in the in-line rolling mill to be selected primarily for optimum control of strip surface roughness.
According to the invention there is provided a method of casting cast steel strip from a casting pool of molten steel using the casting surfaces of a twin roll caster to produce strip of differing thicknesses in the as-cast condition, comprising:
(a) determining for each desired thicknesses of the as-cast strip, a target casting speed which will avoid over-heating of the casting roll surfaces;
(b) determining from each target casting speed a target casting pool depth to produce a cast strip of the desired thickness when the twin roll caster is operated at the target casting speed; and
(c) operating the caster to cast strip based on the determined target casting speed and the determined target depth to produce cast strip generally of the desired thickness.
The method may be performed with a single, twin, or multi-roll roll caster. The as-cast strip may have differing thicknesses, which may be customer-specified, or may be reduced, as by for example in-line rolling, to a desired customer-specified thickness.
In determining the target casting speed and the target casting pool depth, predetermined characteristics of the casting rolls of the roll casters such as the diameter of the casting rolls and heat flux rate through the casting surfaces may be factors to be considered. The casting rolls may include copper or copper alloy sleeves defining the casting surfaces of the rolls. In this case, the casting roll characteristics may include the diameter of the rolls and the thickness of the sleeves, which affect the relation between the casting speed and the casting surface temperature for a particular heat flux.
If these physical characteristics of the casting rolls remain essentially the same, then the caster can be operated at substantially the same production throughput rate, hence it is possible to calculate the target casting speed (u) for a given cast thickness, and then the target casting pool depth is varied to control the as-cast thickness of the strip, i.e., the target casting pool depth is decreased to decrease the as-cast thickness of the strip.
The casting pool depth is measured from the nip of the casting roll, where the strip departs from the casting surfaces of the casting rolls, vertically to the level of the casting pool The target pool depth may be determined from the target casting speed in accordance with the following equation:R*sin−1(h/R)=u*d2/(k2)  (Eq. 1)where, h=pool depth (mm),                R=casting roll radius (mm),        d=half strip thickness (mm).        k=roll k-factor (mm/min0.5),        u=casting speed (mm/min).        
The roll k-factor is determined empirically by determining solidification rates in accordance with the formulad′=k√{square root over (t)}where d′ is the half strip thickness, and t is time.
The invention also provides a method of producing a steel strip to customer-specified thickness comprising operating a twin roll caster in the manner defined above either to produce as-cast strip of differing customer-specified thicknesses or to produce as-cast strip of a thickness greater than the customer-specified thickness and then rolling the cast strip in line with the caster to reduce its thickness to the customer-specified thickness.
The as-cast thickness may be greater than the customer-specified thickness by an amount in the range 0% to 30%. Typically the reduction may be of the order of 15%.
The present invention further provides a method of producing steel strip to a customer-specified thickness by casting strip from a casting pool of molten steel using a pair of casting rolls of a twin roll caster and optionally rolling the as-cast strip to reduce its thickness, comprising:
(a) setting a target as-cast strip thickness based on the customer-specified thickness;
(b) determining a target casting speed based on the selected target as-cast thickness and casting roll characteristics while avoiding over heating of the casting roll surfaces;
(c) determining from the target casting speed a target pool depth to produce a strip of the target thickness when the casting rolls are operated to cast the strip at the target casting speed;
(d) operating the twin roll caster to cast strip based on the target casting speed and the target pool depth; and
(e) optionally in-line rolling the as-cast strip delivered from the caster to reduce its thickness to the customer-specified thickness.
The certain factor for setting the desired as-cast strip thickness may be chosen such that in-line rolling achieves a surface roughness target. The desired as-cast strip thickness may be the customer-specified thickness.